Typically, a conventional thick-film type thermal printhead has an arrangement shown in FIGS. 9-13. Specifically, the thermal printhead generally indicated by reference numeral 10" includes a heat sink plate 20". The heat sink plate is made of a metal material having high thermal conductivity like aluminum. The printhead also includes an elongated rectangular head substrate 11" carried by the heat sink plate 20". The substrate is made of an insulating material such as alumina ceramic.
The head substrate 11" includes a first longitudinal edge 11a" and a second longitudinal edge 11b" opposite to the first longitudinal edge 11a". The head substrate 11" has an upper surface formed with a linear heating resistor 12" extending along the first longitudinal edge 11a". The upper surface is also formed with an array of plural drive ICs 13" arranged along the second longitudinal edge 11b" for actuating the heating resistor 12".
As shown in FIG. 10, the upper surface of the head substrate 11" is formed with a common electrode 14" having comb-like teeth 14a" adjacent to the heating resistor 12". The teeth 14a" extend beneath the heating resistor 12". Further, individual electrodes 15" are provided in an alternating manner relative to the teeth 14a" of the common electrode 14". The individual electrodes 15" also extend beneath the heating resistor 12". The heating resistor 12" is divided into portions defined by adjacent teeth 14a" of the common electrode 14". Each portion (see the shaded area in FIG. 10) operates as a heating dot 16". When voltage is selectively applied on the individual electrodes 15" via the drive ICs 13", relevant heating dots 16" will be actuated for heating.
As shown in FIG. 12, each individual electrode 15" extends toward the second longitudinal edge 11b" of the head substrate 11" to be connected to the output side of a corresponding drive IC 13" via a bonding wire 21a". The input side of each drive IC 13" is connected via a bonding wire 21b" to a wiring pattern 22" formed on the head substrate 11". The bonding wires 21a", 21b" together with the drive ICs 13" are enclosed by a protective coating 17" made of an epoxy resin.
The conventional protective coating 17" is formed in the following manner. While being shifted, a dispenser having a projection nozzle supplies a viscid but fluid epoxy resin to enclose the drive ICs 13" and the bonding wires 21a", 21b". Then, the substrate 11" is brought into a heating furnace to cure the above epoxy resin.
In the field of thermal printheads of the type described above, efforts have been made to minimize the size of the thermal printheads. More specifically, the longitudinal length of the head substrate 11" is inevitably adapted to a desired printing span. Thus, efforts have been made to minimize the widthwise dimension of the head substrate 11". Accordingly, the protective coating 17" needs to be properly formed within a limited region as viewed widthwise of the substrate. To this end, the epoxy resin to be utilized is selected from resins which have comparatively high viscosities when supplied from the dispenser. This is because an epoxy resin of a low viscosity would disadvantageously flow onto unintended regions in its application.
In utilizing an epoxy resin material having high viscosity, the application of the resin material needs be performed along a spiral path as shown in FIG. 11. Specifically, starting from a point adjacent to one end of the array of the drive ICs 13", the resin application is first performed for the bonding wires 21a" which connect the drive ICs 13" and the individual electrodes 15" (see also FIG. 12). Then, turning at the opposite end of the array of the drive ICs 13", the resin application is performed for the bonding wires 21b" which connect the drive ICs 13" and the wiring pattern 22". Further, turning at the first-mentioned end of the array of the drive ICs 13" while also being shifted inward, the resin application is performed for the drive ICs 13". At this time, the applied resin is arranged to extend over the array of the drive ICs 13" longitudinally thereof. Resin application is performed in the above-described spiral manner. This is because, if the application of the resin material for the mounting region of the drive ICs 13" is performed only once along a single straight line, the predetermined area needed to be applied by the resin material will not be entirely covered by the resin material due to the comparatively high viscosity of the epoxy resin. Further, the spiral application path is preferable for causing the resulting protective coating 17" to have a suitable cross section.
As shown in FIG. 11, the resin application path begins at one end of the array of the drive ICs 13" and terminates at the opposite end thereof. Further, as shown in FIG. 13, a horn-like protrusion 17a" may be formed at the terminal end of the resin application path. This is because the applied epoxy resin has the rather high viscosity and, at the terminal end of the application path, the projection nozzle of the dispenser is being shifted upward after the resin supply is stopped. Then, the protrusion 17a" will be cured with the horn shape maintained.
Thus formed protrusion 17a" of the protective coating 17" may unfavorably damage a recording medium such as recording paper or deteriorate prints formed thereon through contact with the recording medium. Such inconvenience will become more critical to the latest model of e.g. printing device, in which the feeding path of recording paper is disposed as close to the surface of the thermal printhead as possible for purposes of miniaturization for example.